Changes to pressure module

[timcallow]

In this PR, we introduce more options for calculating the pressure, i.e.:

• Finite differences can be computed with different energy formulations
• Stress-tensor method has option for radial-only component
• Virial method has use_correction option
• Adds ideal electron pressure function

[codecov[bot]]

Codecov Report

Merging #175 (2b3e4c4) into develop (fd266db) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #175      +/-   ##
===========================================
+ Coverage    97.64%   97.70%   +0.06%     
===========================================
  Files           14       14              
  Lines         2205     2226      +21     
===========================================
+ Hits          2153     2175      +22     
+ Misses          52       51       -1     

Impacted Files	Coverage Δ
atoMEC/models.py	96.85% <ø> (ø)
atoMEC/postprocess/pressure.py	97.56% <100.00%> (+0.83%)	:arrow_up:

... and 1 file with indirect coverage changes

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